Radio Frequency (RF) receivers of the Zero Intermediate Frequency (Zero-IF) and Low Intermediate Frequency (Low-IF) type have a first signal path for an In-phase (I) signal and a second signal path for a Quadradure (Q) signal. A received signal is mixed with a Local Oscillator (LO) signal to generate the I signal and with the same LO signal shifted in phase by 90° to generate the Q signal, but in all other respects the signal paths should be the same. Unfortunately, in a real receiver, it is very difficult to ensure that the phase shift is exactly 90°. It is also inevitable that there are differences between the components of the first and second paths that lead to variations in the delay and gain experienced by the signals along the signal paths. This problem is generally referred to as “IQ mismatch”.
In general, in an integrated circuit (IC), any difference in delay along the signal paths is not very large, and this causes less of a problem than variations in the gain and phase shift. However, the variations in gain and phase shift lead to distortion of the I and Q signals, creating additional noise and limiting reception performance.
Various ways of compensating for IQ mismatches have been proposed. For example, the IQ mismatches can be estimated and corrections applied to the I signal and the Q signal based on the estimates. In other methods, the IQ mismatches can be treated as noise or interference and steps taken to cancel this from the signals. However, no existing method is ideal. In particular, some methods perform well for relatively low mismatches, but less well for higher mismatches. Indeed, some such methods can fail altogether when mismatches reach a certain level. Other methods react too slowly to deal with fast changes in the IQ mismatches.
The paper “Blind IQ Signal Separation-Based Solutions for Receiver Signal Processing”, Valkama et al, EURASIP Journal on Applied Signal Processing, 1 Jan. 2005, pp 2708-2718 describes a technique for compensating for IQ mismatches that uses Blind Signal Separation (BSS). BSS avoids the need for training signals, and instead relies on analysing the I and Q signals themselves as a radio signal is received. However, the technique described in this paper does not cope well when IQ mismatches change quickly.
There is therefore a need for improved IQ mismatch compensation.